1. Field of Invention
The present invention pertains to a method of etching silicon oxynitride and other oxygen-containing materials. Silicon oxynitride is primarily used as an antireflective coating (ARC) and is often referred to as a xe2x80x9cdielectric ARCxe2x80x9d. Silicon oxynitride is frequently used in combination with deep ultraviolet (DUV) photoresists.
2. Brief Description of the Background Art
In the field of semiconductor device fabrication, DUV photoresists have been developed which take advantage of shorter wavelengths of ultraviolet radiation to enable the patterning of smaller electronic and optical devices than possible with traditional, or so called I-line, photoresists. Generally, the photoresist is applied over a stack including layers of various materials to be patterned in subsequent processing steps. Some of the layers in the stack are consumed during the process of patterning underlying layers which become part of the functioning device. To take advantage of the spacial resolution of the photoresist, it is necessary to use an antireflective coating (ARC) layer underlying the photoresist in order to suppress reflection off other layers in the stack during photoresist exposure. Thus, the ARC layer enables patterning of the photoresist to provide an accurate pattern replication.
Though the most commonly used ARC material is titanium nitride, a number of other materials have been suggested for use in combination with DUV photoresists. For example, U.S. Pat. No. 5,441,914, issued Aug. 15, 1995 to Taft et al., describes the use of a silicon nitride antireflective layer, while U.S. Pat. No. 5,525,542, issued Jun. 11, 1996 to Maniar et al., discloses the use of an aluminum nitride antireflective layer. U.S. Pat. No. 5,539,249, issued Jul. 23, 1996 to Roman et al., describes the use of an antireflective layer of silicon-rich silicon nitride.
U.S. Pat. No. 5,635,338, issued Jun. 3, 1997 to Joshi et al., describes a class of silicon-containing materials which display particular sensitivity in the ultraviolet and deep ultraviolet for the formation of patterns by radiation-induced conversion into glassy compounds. Joshi et al. recommend the use of antireflective coatings such as amorphous silicon and an organic plasma polymerized antireflective coating generated from cycloheptatriene.
U.S. Pat. No. 5,633,210, issued May 27, 1997 to Yang et al., discloses the use of an antireflective coating material selected from titanium nitride materials, silicon oxide materials, and silicon oxynitride materials.
Recently, there has been increased interest in the use of silicon oxynitride as an antireflective coating due to its ability to function well in combination with DUV photoresist. Silicon oxynitride typically (but not by way of limitation) has a formula of SiOxNyHz, where x ranges from 0 to about 2, y ranges from 0 to about 1, and z ranges from 0 to about 1. By changing the composition of the silicon oxynitride ARC layer, one can control reflection onto the photoresist during imaging of the photoresist layer. When SiOxNyHz is used as an ARC, x, y, and z typically range between about 0.2 and about 0.5.
Silicon oxynitride as an ARC enables efficient suppression of the reflection from underlying layers, while providing superior chemical properties which prevent an undesirable effect, known as photoresist poisoning, in photoresist patterning. Photoresist poisoning refers to reaction of the surface underlying the photoresist with moisture to form amino basic groups (NH2xe2x88x92) which react with the photogenerated acid which is responsible for the photoresist development. Deactivation of the acid by the amino groups is believed to be responsible for formation of the xe2x80x9cfootxe2x80x9d (widening of the photoresist line just above the substrate) on some ARC materials, such as titanium nitride.
The present invention addresses details of the application of dry etch techniques for pattern transfer into a silicon oxynitride layer. However, the concepts developed for dry etch of a silicon oxynitride layer have application to the dry etch of other oxygen-containing substrates.
With reference to a silicon oxynitride layer used as an antireflective coating, in such an application, a typical stack of materials for pattern transfer would include (from bottom to top): A substrate, which is a dielectric layer used to separate a metal interconnect layer (to be patterned on plasma etching of the etch stack) from underlying layers of the integrated circuit; a barrier layer, which prevents the diffusion of material between a conductive layer and the substrate; a conductive layer, which is typically aluminum or an alloy thereof; an antireflective coating (ARC) layer, which reduces reflection back into the photoresist during its exposure in the lithography step and allows for better pattern reproduction; and, a photoresist layer, which is imaged to provide the pattern for transfer to underlying layers.
It would then be desirable to have a dry, plasma-based etch process for transfer of the pattern from the developed photoresist through all of the layers within the complete etch stack, including an ARC layer, a conductive layer, and a barrier layer. Etching of a metal-comprising stack is traditionally achieved in a metal etch chamber using etch stacks with ARC layers such as titanium nitride. However, as silicon oxynitride is a dielectric material, its patterning is traditionally reserved for dielectric etch chambers used for etching oxide and nitride. As a result, the substrate is typically moved from one process chamber to another, which lowers the overall productivity of the whole process.
The present invention details a method permitting the etch of a dielectric-comprising ARC layer, such as a silicon oxynitride ARC, in the same chamber as is used for etching the rest of the metal-comprising stack. We have developed a plasma etch process which provides adequate selectivity for etching a silicon oxynitride ARC layer over organic-based photoresists. In addition, we have obtained a good etch rate for a silicon oxynitride ARC layer, while providing excellent pattern transfer through the ARC layer and other layers of a six-layer, metal-comprising stack. Further, the method of the invention solves a series of integration problems stemming from the fact that the chemistry used for etching the silicon oxynitride ARC layer is very different from that used in the metal etch.
The present invention pertains to a method for plasma etching a semiconductor film stack. The film stack includes at least one layer comprising an oxygen-containing compound. In one preferred embodiment of the method, the chemistry enables the plasma etching of both a layer of oxygen-comprising material and an adjacent or underlying layer of a different material. In another preferred embodiment of the method, the layer of oxygen-comprising material is etched using one chemistry, while the adjacent or underlying layer is etched using another chemistry, but in the same process chamber. Of particular interest is silicon oxynitride, an oxygen-comprising material which functions as an antireflective material.
A preferred embodiment of the method provides for the use of a source of carbon and an appropriate halogen-comprising plasma to achieve selective etch of one oxygen-containing material compared with another material which contains a more limited amount of oxygen.
In a highly preferred embodiment of the invention, a film of silicon oxynitride is plasma etched, and better selectivity of etching is achieved relative to a film of a lower oxygen content material, such as a photoresist, by using a fluorine-comprising plasma. Preferably, the fluorine-comprising plasma also comprises a source of carbon. Examples of plasma feed gases which provide both fluorine and carbon include fluorocarbons such as CHF3, CF4, CF3Cl, C2F4, C2F6, and combinations thereof. The fluorocarbon gases may be combined with other gases which increase the halogen content of the plasma, such as Cl2, F2, HF, HCl, NF3, or SF6, for example, but not by way of limitation. The addition of such other gases is helpful in increasing the etch rate of, and in some instances the etch selectivity toward, the silicon oxynitride. When the gas used to increase the halogen content comprises a halogen other than fluorine, such as chlorine, the etch rate of some other stack materials, such as a TiNx barrier layer, is also increased. The addition of chlorine to a fluorocarbon-containing plasma should enhance etch of such a barrier layer material along with the silicon oxynitride, while the etch of an oxygen-poor material such as a photoresist is suppressed.
We have discovered a preferred combination of plasma etch gases which provides an unexpectedly high etch rate, while providing selectivity toward etching the silicon oxynitride over patterning photoresist. The preferred etchant gas mixture includes chlorine and at least one compound comprising fluorine and carbon. The atomic ratio of fluorine to chlorine in the etchant gas mixture ranges between about 3:1 and about 0.01:1. A ratio of about 3:1 fluorine:chlorine is recommended for high silicon oxynitride to photoresist etch selectivity. It is expected that the use of CF4 rather than CHF3 would require less Cl2 to selectively etch silicon oxynitride and that CF4 alone is likely to be sufficient. Further, since the silicon etch rate is dependent on fluorine rather than chlorine, the use of CF4 should increase the etch rate of silicon oxynitride and may provide an improvement in etch selectivity as compared with CHF3.
A higher chlorine content is recommended when etching both the layer of silicon oxynitride and an underlying layer containing a metal or a refractory metal such as titanium nitride. A preferred etchant gas mixture has an atomic ratio of fluorine to chlorine between about 0.5:1 and about 0.01:1, most preferably, between about 0.25:1 and about 0.1:1.